Effects of extraction methods and factors on leaching of metals from recycled concrete aggregates

Abstract

Leaching of metals (calcium (Ca), chromium (Cr), copper, (Cu), iron (Fe), and zinc (Zn)) of recycled concrete aggregates (RCAs) were investigated with four different leachate extraction methods (batch water leach tests (WLTs), toxicity leaching procedure test (TCLP), synthetic precipitation leaching procedure test (SPLP), and pH-dependent leach tests). WLTs were also used to perform a parametric study to evaluate factors including (i) effects of reaction time, (ii) atmosphere, (iii) liquid-to-solid (L/S) ratio, and (iv) particle size of RCA. The results from WLTs showed that reaction time and exposure to atmosphere had impact on leaching behavior of metals. An increase in L/S ratio decreased the effluent pH and all metal concentrations. Particle size of the RCA had impact on some metals but not all. Comparison of the leached concentrations of metals from select RCA samples with WLT method to leached concentrations from TCLP and SPLP methods revealed significant differences. For the same RCA samples, the highest metal concentrations were obtained with TCLP method, followed by WLT and SPLP methods. However, in all tests, the concentrations of all four (Cr, Cu, Fe, and Zn) metals were below the regulatory limits determined by EPA MCLs in all tests with few exceptions. pH-dependent batch water leach tests revealed that leaching pattern for Ca is more cationic whereas for other metals showed more amphoteric. The results obtained from the pH-dependent tests were evaluated with geochemical modeling (MINTEQA2) to estimate the governing leaching mechanisms for different metals. The results indicated that the releases of the elements were solubility-controlled except Cr.

Toxicity of Recycled Concrete Aggregates: Review on Leaching Tests

“…Bestgen [1] indicates in its study that heavy metals concentrations of the RCA's eluate do not show a correlation with their content in the constitution of the aggregates. As an example, the author indicates that RCA with lower concentrations of Cu (%) corresponds to eluate with a greater concentration of the same metal.…”

“…Some authors have analysed the relation between L/S and time [25] and between L/S ratio and saturation degrees [1]. According to some authors, the L/S ratio variation in percolation experiments is intended to simulate different degrees of saturation to which aggregates may be subjected in reality and to interpret how this parameter may affect substances leaching [1] [27].…”

“…Bestgen [1] stated that a decrease in pH occurs after adding water to the RCA. Another characteristic that affects the aggregates' pH is their reduction to sizes defined in the standardization.…”

“…Depending on their origin, Recycled Concrete Aggregates (RCA) from Construction and Demolition Waste (CDW) have different characteristics [1]. If the recycling plant does not carry out adequate management, CDW can be contaminated, impairing their recovery as aggregates [2,3].…”

Neutralization of high pH and alkalinity effluent from recycled concrete aggregate by common subgrade soil

“…Alkaline wastes, such as RCA, fly ash, and red mud, have been extensively reused as engineering fills or additives in concrete in recent years (Abbaspour, Tanyu, & Cetin, 2016;Bestgen, Cetin, & Tanyu, 2016;Cetin, Aydilek, & Li, 2014;Gomes, Mayes, Rogerson, Stewart, Burke, 2016;Sun et al, 2019). Leachate generated from these wastes are often associated with the concerns of high pH (usually pH = 9-14) and elevated concentrations of hazardous elements, especially oxyanions (e.g., arsenic [As] and chromium [Cr]) (Chen, Bradshaw, Benson, Tinjum, & Edil, 2012;Sun et al, 2019).…”